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We have re-evaluated empirical expressions for the abundance determination ofN, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data andconstructing an appropriate grid of photoionization models with state-of-theart model atmospheres. Using these expressions we have derived heavy elementabundances in the $\sim$ 310 emission-line galaxies from the Data Release 3 ofthe Sloan Digital Sky Survey (SDSS)with an observed Hbeta flux F(Hbeta)> 1E-14erg s^{-1} cm^{-2} and for which the [O III] 4363 emission line was detected atleast at a 2sigma level, allowing abundance determination by direct methods.The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ~7.1 (Zsun/30) to 8.5 (0.7 Zsun). The SDSS sample is merged with a sample of 109blue compact dwarf (BCD) galaxies with high quality spectra, which containsextremely low-metallicity objects. We use the merged sample to study theabundance patterns of low-metallicity emission-line galaxies. We find thatextremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Zsun/12) are rarein the SDSS sample. The alpha element-to-oxygen abundance ratios do not showany significant trends with oxygen abundance, in agreement with previousstudies, except for a slight increase of Ne/O with increasing metallicity,which we interpret as due to a moderate depletion of O onto grains in the mostmetal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value,by up to 1 dex at the high metallicity end. (abridged)Comment: 17 pages, 12 figures. Accepted for publication in the Astronomy and  Astrophysic

The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky Survey